Modern technology is focusing increasingly on portable electronic devices, not only in the consumer realm but in the industrial and military realm as well. Such portable devices rely on portable energy sources, and require ever-increasing energy storage capacity. The three major energy storage devices are batteries, capacitors, and supercapacitors, which bridge the characteristics of a batteries and capacitors. The billion-dollar supercapacitor market is expected to increase rapidly as portable electronic devices expand into new consumer, industrial, and military applications.
A supercapacitor can store energy through a capacitive process, such as an electrochemical double layer capacitor (EDLC), or through a Faradic battery-like reaction termed a pseudocapacitor.
An EDLC generates a double layer capacitance by charge separation at an interface between an electrolyte and a high surface area electrode such as activated carbon, carbon nanotubes (CNTs) or similar materials. See A. Fischer et al., “Incorporation of Homogeneous, Nanoscale MnO2 within Ultraporous Carbon Structures via Self-Limiting Electroless Deposition: Implications for Electrochemical Capacitors,” Nano Letters, Vol. 7, No. 2, pp. 281-286 (2007); and A. Fischer et al., “Electroless Deposition of Nanoscale MnO2 on Ultraporous Carbon Nanoarchitectures: Correlation of Evolving Pore-Solid Structure and Electrochemical Performance,” J. Electrochem. Soc., Vol. 155, No. 3, pp. A246-A252 (2008). The EDLC has large power densities by the fast absorption/desorption kinetics of protons but the energy density is limited. See I. H. Kim, et al., “Synthesis and Electrochemical Characterization of Vanadium Oxide on Carbon Nanotube Film Substrate for Pseudocapacitor Applications,” J. Electrochem. Soc., Vol. 153, No. 6 pp. A989-996 (2006); and Y. T. Kim, et al., “Highly dispersed ruthenium oxide nanoparticles on carboxylated carbon nanotubes for supercapacitor electrode materials,” J. Mater. Chem., Vol. 15, pp. 4914-4921 (2005).
In contrast, a pseudocapacitor operates by an electrochemical reversible Faradaic redox reaction at a solid electrode of a conducting polymer or a metal oxide. The fast redox reaction gives the pseudocapacitor superior energy density but at a lower power density compared to an EDLC. I. H. Kim, supra; Y. T. Kim, supra.
Kalpana et al. proposed combining the double layer capacitance of large surface area nano-carbon aerogel with the Faradaic capacitance of zinc oxide (ZnO). See D. Kalpana et al., “A novel high power symmetric ZnO/carbon aerogel composite electrode for electrochemical supercapacitor,” Electrochimica Acta, Vol. 52, pp. 309-1315 (2006). This carbon aerogel/ZnO composite showed high electrochemical reversibility and capacitive characteristics in a KOH electrode.
The highest power density pseudocapacitor was constructed with hydrated ruthenium oxide (RuO2) in a highly acidic sulfuric acid electrolyte. See J. P. Zheng et al., “Hydrous Ruthenium Oxide as an Electrode Material for Electrochemical Capacitors,” J. Electrochem. Soc., Vol. 142, No. 8, pp. 2699-2703 (1995); and C. C. Hu et al., “Effects of preparation variables on the deposition rate and physicochemical properties of hydrous ruthenium oxide for electrochemical capacitors,” Electrochimica Acta, Vol. 46, pp. 3431-3444 (2001).
However, the high cost of ruthenium and a desire to operate in a neutral electrolyte has positioned manganese oxide as a cost-effective solution. See M. Toupin et al., “Influence of Microstructure on the Charge Storage Properties of Chemically Synthesized Manganese Dioxide,” Chem. Mater., Vol. 14, pp. 3946-3952 (2002); and Y. U. Jeong et al., “Nanocrystalline Manganese Oxides for Electrochemical Capacitors with Neutral Electrolytes,” J. Electrochem. Soc., Vol. 149, No. 11, pp. A1419-A1422 (2002).
The reduction-oxidation reaction with the manganese oxide layer has a limited stability regime in an electrochemical-type cell. The upper voltage is limited by the onset of a non-reversible redox oxygen evolution process and the lower voltage is limited by the non-reversible reduction and dissolution of the manganese ion. Therefore, the most useful design is as an asymmetric device with an activated-carbon paper negative electrode and a pseudocapacitve manganese oxide-carbon paper positive electrode. A nanoscopically thin manganese oxide coating in close proximity to the highly conductive carbon minimizes the impact of the low conductivity of manganese oxide. See J. W. Long, “Electrochemical Capacitors emPOWERING the 21st Century,” The Electrochemical Society Interface, p. 33 (2008).
Despite the advantages of using a porous carbon paper electrode, however, deposition of a nanometer-scale thick layer of a metal oxide such as manganese oxide conformally throughout the entire high-surface area carbon paper is extremely challenging.